Cleaning methods have been proposed in the art for maintaining the operation of optical windows and/or sensing membranes in remote and/or fouling environments. For example, the patent application Ser. No. GB 2379976 to Benson describes an optical cell in which the outer part of the optical window that is in contact with a material being analyzed may be rotated and passed over a wiper. Benson further provides that a cleaning fluid can be used with the optical window to aid the removal of contaminants from the portion of the window rotated for cleaning and warm air may be used to dry the window prior to it being rotated back into its original position. Similarly, U.S. Pat. No. 6,627,873 to Tchakarov et al. describes the use of wiper blades to maintain the transmission properties of optical windows used to measure the concentration of carbon dioxide in fluids produced from subsurface formations by means of an infrared gas sensor.
With regard to acoustic sensors and/or acoustic resonators, the use of acoustic devices to measure organic and inorganic deposits associated with wellbores for producing hydrocarbons is described in the commonly owned '402 and '406 patents. Additionally, these two patents disclose using such acoustic devices in proximity to optical sensors to provide for cleaning the optical sensors and/or using spectroscopic-type measurements from the optical sensors in combination with the acoustic-type measurements from the acoustic devices to measure properties of deposits associated with the hydrocarbon production.
Furthermore, the commonly owned '360 patent describes embedding a diode and a detector of a mid-infrared gas sensor in separate acoustic devices that are located down a borehole—the diode and detector being used to measure properties of a gas located between the diode and the detector—and using the acoustic devices to provide for acoustic cleaning of the diode and the detector. Additionally, the GB 2336668 patent to Byatt et al. discloses the use of fluorescence spectroscopy to determine the concentration of oil in water and mentions that fouling of the optical windows might be prevented or minimized by the use of oil-repellent coatings, turbulent flow or application of ultrasound.
U.S. Pat. No. 6,437,326 to Yamate and Mullins may be understood to suggest the use of ultrasonic oscillators to clean optical windows in optical sensors placed permanently in wells producing hydrocarbons. U.S. Pat. No. 6,426,974 discloses the application of high frequency ultrasound to the cleaning of a flow cell that is designed for use in a light scattering apparatus. The WO2004/003506 application to Pope et al. provides for an optical sensor to measure the concentration of methane gas that can be produced from coal beds, wherein an acoustic cleaner, that is separate from an optical window, may be used to ultrasonically clean the optical window. Likewise, U.S. Pat. No. 4,216,671 to Kurland discloses the use of transducers to clean sensor membranes that are separate from the transducers. Furthermore, Windsor Scientific Ltd in the United Kingdom manufacture what is known as a sonotrode, which is a carbon electrode that is attached by glass rods to an acoustic resonator.
U.S. Pat. No. 4,956,149 to Kawana and Ito describes a biosensor combined with a piezoelectric actuator to dispense liquid drops with highly reproducible volumes. GB2269674 to Campbell discloses an electrochemical membrane sensor containing a piezoelectric transducer for the purposes of cleaning the membrane and the attached electrode and of mixing the electrolyte contained in the sensor. In addition, U.S. Pat. No. 5,889,209 to Piedrahita and Wong discloses the use of ultrasound from an ultrasonic transducer to clean a sensor's membrane, wherein the ultrasonic transducer and the sensor are placed adjacent to each other.